Means for generating power, such as electric motors, internal combustion engines, human power, and the like, typically generate power most efficiently when they operate within a relatively narrow range of speeds, while a specific applications for the power might require a broad range of speeds, e.g., a wheeled vehicle. A mechanical transmission is a device interposed between a means for generating power and a specific application for the purpose of adapting the speed and power characteristics of one to the other.
Most mechanical transmissions function as rotary speed changers, where the ratio of the output speed of the transmission to the input speed may be constant, e.g., a gear box, or variable.
A distinction which can be drawn among various variable speed transmissions is that between automatic and manually controlled transmissions. In manual transmissions, the gear ratio is manually changed by the operator, while in automatic transmissions, the gear ratio changes in response to the load on the transmission. Manual transmissions offer the advantage of putting the operator in complete control of the timing and value of the gear ratio selection, however, the shifting process may serve to distract the operator from more pressing aspects of the operation of the machine.
A further distinction which can be drawn among various variable speed drives is that between those drives where the speeds are variable in discrete steps, i.e., stepped transmissions, e.g., a classical bicycle transmission employing a derailer, or those which are continuously variable within a specified range, i.e., stepless drives. Step-variable transmissions usually employ either gears or chains and provide fixed speed ratios with little or no slip. Stepless transmissions use either belts, chains, or rolling contact bodies. Stepped transmissions have a number of important drawbacks: (i) the power source is rarely operating at its optimum speed, (ii) there is a momentary disengagement of the drive as it is shifting between the steps of the transmission which can lead to output power interruptions, and, (iii) there is significant wear on the components of the transmission due to the discontinuous load variation during shifting, particularly when under a heavy load.
A variety of designs exist for continuous stepless transmissions. A widely used and inexpensive stepless drive consists of a V-belt running on variable-diameter pulleys. The sides of the pulleys are conical on the inside to match the taper of the V-belt, and moving them closer together causes the V-belt to move outward from the center of the pulley thereby operating on a larger effective circle, thus changing the speed ratio. Such drives have the drawback of being dependent on friction between the V-belt and the pulley and are therefore subject to power loss due to slippage, i.e., they are not positive-engagement drives.
Stepless transmissions employing rolling contact bodies are known as traction drives. In these transmissions power is transmitted in a variety of ways that depend on the rolling friction of bodies in the form of cylinders, cones, balls, rollers, or disks. Again, because these drives depend on friction to transmit power, they are subject to power loss due to slippage. Furthermore, theses drives tend to be mechanically bulky and therefore not suited to light-duty applications.
For the foregoing reasons, there is a need for a mechanical transmission which (i) is adaptable to both manual and automatic operation, (ii) provides a continuous selection of gear ratios over a specified range, (iii) is not subject to power loss due to slippage, and (iv) is suitable to both heavy and light-duty applications such as motor cycles, winches, bicycles, and other like human-powered vehicles.